Sleep mode can go by many different names, including Stand By, Sleep, and Suspend. When placed in this sleep mode, aside from the RAM which is required to restore the machine's state, the computer attempts to cut power to all unneeded parts of the machine. Because of the large power savings, most laptops automatically enter this mode when the computer is running on batteries and the lid is closed.

There is a variety of mechanisms to enable your operating system to suspend to memory or to disk. To understand the difference between these systems, you need to know that there exists a suspend/resume implementation in the kernel (swsusp) and typically a set of additional tweaks to handle specific drivers/modules/hardware (e.g. video card re-initialization).

systemd provides commands for suspend to RAM, hibernate and a hybrid suspend using the kernel's native suspend/resume functionality. There are also mechanisms to add hooks to customize pre- and post-suspend actions.

pm-utils is a set of shell scripts that encapsulate the kernel's suspend/resume functionality. It comes with a set of pre- and post-suspend tweaks and various hooks to customize the process.

uswsusp also aims to provide programs that encapsulate the kernel's suspend/resume functionality with the additional necessary tweaks. It also aims to provide a suspend-to-both functionality - this allows resuming from memory if battery is not depleted and resuming from disk if battery is completely depleted.

TuxOnIce differs from pm-utils and uswsusp in that it attempts to directly patch the kernel's suspend/resume functionality to add more functionality than the default implementation. Therefore it requires a custom kernel.

Note that the end goal of these packages is to provide binaries/scripts that can be invoked to perform suspend/resume. Actually hooking them up to power buttons or menu clicks or laptop lid events is left to other mechanisms. To automatically suspend/resume on certain power events, such as laptop lid close or battery depletion percentage, you may want to look into running Acpid.

Suspend methods

These methods can be used to suspend/resume directly. pm-utils is also fairly generic, so its pm-suspend and pm-hibernate scripts can be configured to use any of these methods.

kernel

The most straightforward approach is to directly inform the in-kernel software suspend code (swsusp) to enter a suspended state; the exact method and state depends on the level of hardware support. On modern kernels, writing appropriate strings to /sys/power/state is the primary mechanism to trigger this suspend. See http://acpi.sourceforge.net/documentation/sleep.html for details.

In general, it is possible that just installing pm-utils and invoking pm-suspend (which uses the kernel backend by default) just works for you. In some cases, you may need to force specific modules to be unloaded to make this work, as described in pm-utils#Advanced Configuration.

systemd

See Power Management#Sleep hooks for additional information on configuring suspend/resume hooks. Also see man systemctl, man systemd-sleep, and man systemd.special.

uswsusp

The uswsusp ('Userspace Software Suspend') package provides s2ram, a wrapper around the kernel's suspend-to-RAM mechanism which perform some graphics adapter manipulations from userspace before suspending and after resuming. See Uswsusp.

tuxonice

TuxOnIce is a fork of the kernel implementation of suspend/resume that provides kernel patches to improve the default implementation. It requires a custom kernel to achieve this purpose. Since pm-utils is a set of shell scripts with a variety of hooks, it can be configured to use TuxOnIce as well.

pm-utils configuration

Deciding between these options

Pm-utils framework or not?

Directly calling the kernel backend method is significantly faster than calling pm-suspend, since running all the hooks provided by the pm-utils framework invariable takes time. Even uswsusp is faster than pm-suspend. However, the recommended approach is to use pm-utils as it can properly set hardware clock, restore wireless etc.

Selecting the backend/method

kernel - hooks provided by pm-utils (including video quirks) with kernel method. This is the recommended mechanism. It may require specific kernel modules to be unloaded before it will work properly. Searching on the arch linux bbs for your specific laptop is a good idea to discover these modules.